Diabetes mellitus is a chronic disorder characterized by insulin deficiency, hyperglycemia, and high risk for development of complications of the eyes, kidneys, peripheral nerves, heart and blood vessels. The disease is highly prevalent, affecting nearly 26 million people in the U.S. The landmark nine-year Diabetes Control and Complications Trial (DCCT), completed in 1993, showed conclusively that the risk for development and progression of the chronic complications in patients with type 1 diabetes mellitus could be dramatically reduced with improved blood glucose control as assessed by serial HbA1c determinations. HbA1c measurement standardization has become even more important in recent years for two reasons; first, there is increased use of the test, with rising rates of diabees and second, the test has recently been recommended for use in diabetes diagnosis. In addition, the FDA has utilized improvements in HbA1c as an outcome measure for approval of new therapies for diabetes. Thus there is now a critical need for accuracy and precision near the normal range in addition to the clinical target range for diabetes treatment. DCCT established the importance of another key laboratory measurement in type 1 diabetes. C-peptide is a stable and detectable non-functional cleavage product of insulin found in serum. Measurement of C-peptide is used to monitor endogenous insulin production in people with diabetes who are receiving exogenous insulin as therapy. There is increasing evidence, including data from the DCCT study, that preservation of even a low level of endogenous insulin production in people with type 1diabetes is associated with significantly fewer diabetes complications. Measuring C-peptide levels with highly sensitive and standardized methods is therefore important now to accurately implement and interpret clinical trials to identify therapeutic interventions capable of reversing or delaying progression of the disease at onset. Improving, harmonizing, and standardizing C-peptide measurements could be even more important in the future as additional interventions and combination of agents are tested for clinical benefit. This application if for continued implementation of standardization programs for both HbA1c and C-peptide. PUBLIC HEALTH RELEVANCE: The importance of HbA1c as a marker of glycemic control in diabetes mellitus, and more recently as a diagnostic tool, has been established. This proposed project will improve the measurement of HbA1c for optimal clinical and diagnostic use. Preservation of beta cell function in type 1diabetes has been identified as an important goal in delaying progression of the disease at onset and also for therapeutic intervention. Accurate, standardized measurement of C- peptide will aid in this goal and is also the subject of this application.